Distillation is the most widely employed separation means in the chemical industry. Distillations are conducted within distillation columns having mass transfer contacting elements such as sieve trays, structured packing and random packing. The contacting elements contact ascending and descending phases of the mixture to be separated such that the ascending vapor phase becomes richer in the more volatile components as it ascends within the column and the liquid phase becomes richer in the less volatile components as the liquid phase descends in the column. In case of air separation, in for example, a double column arrangement of high and low pressure columns connected to one another in a heat transfer relationship, the vapor phase becomes richer in nitrogen and the liquid phase becomes richer in oxygen to effect a separation between nitrogen and oxygen.
Compared to typical industrial distillation practice, cryogenic air separation is characterized by the need for tall distillation columns encased in high efficiency insulation. The high height results from both the large amount of equilibrium stages required to efficiently effect the separation and the fact that in the most common embodiment two columns are stacked one on top of the other to allow thermal integration without the need for a liquid pump. The insulation is required to minimize heat leak from the ambient which increases refrigeration energy requirements and also to prevent ingress of air which would freeze on cold surfaces.
The height required for the distillation columns used in air separation service results in a high incremental cost for the air separation plant. In order to minimize the height requirements, mass transfer contacting elements located within the distillation columns are used that deliver a high separation in a given column height. In case of trays, the high separation requirement is realized by smaller tray spacings than are typically employed in distillation columns used in separating mixtures other than air. Similarly, sheet metal, cross-corrugated structured packings with high area density are used to reduce the height that would otherwise be required for distillation columns used in air separation. One drawback to the use of high density packing is the reduction in hydraulic capacity of the packing. As a result of the reduction in hydraulic capacity, columns with larger cross sectional area are required to process a given amount of air. This increases the quantity of packing required and shell casing costs. It can also lead to shipping constraints. Further, this increase in area results in an increase of interfacial area in sheet metal packings that can result in local maldistribution of liquid between the sheets that can lead to an increase in the height of the packing required to perform a given separation over that suggested by a direct proportion with the increased area (a doubling of the interfacial area does not halve the height required for the separation).
Although, trays and structured packings, as discussed above are the most common type of mass transfer contacting elements that are used in distillation columns, monolithic elements formed of open-cell foams have also been suggested. In such a material, a network of interconnected struts defines an open network of inter connected cells. For example, in U.S. Patent Appln. Ser. No. 2010/0243520, an element is disclosed that can be used in connection with distillation. The element itself can be disc-shaped and stacked into layers within the column. Further, each element can be made up of sub-elements where it is desired to span a large column diameter in much the same manner as slices of a pie. The material making up each element can be an oxide, carbide, nitride, boride, a ceramic material, a metallic material, a polymeric material or a vapor deposition material. The type of material used can be selected to function in harsh environments and silicon carbide is a preferred material.
The effectiveness of a packing made of a foam-like material in a monolithic form within a distillation column is severely limited. This is because the hydraulic capacity of the foam to allow the countercurrent flow of liquid and vapor is low. Further, the material is prone to the promotion of maldistribution where liquid and vapor segregate across the column cross section so as to ease the countercurrent flow of the two phases. It is well known that such segregation is very detrimental to overall effectiveness of mass transfer, particularly in industrial scale columns (>0.3 m diameter).
An inherently more efficient manner of using an open cell foam is replacing the metal sheet in a conventional cross-corrugated structured packing with the foam-like material. The corrugated foam-like structure of corrugated sheets can provide an increase in wetted area versus a conventional sheet packing while also providing gross dimensions open for vapor flow and as such, provide a hydraulic capacity that is comparable or greater than a sheet metal packing of comparable surface area. Such a packing is described in Chinese Patent Application, Serial No. 101555138 A. This patent discloses a method of manufacturing a cross-corrugated structured packing formed of a silicon carbide open cell foam.
It has been determined by the inventor herein that a problem in practically using a cross-corrugated structured packing in the form described above is that the open-cell foam sheets will inherently be more resistive to vapor flow than the area between the sheets. Consequently, in such a packing, vapor will tend to flow at higher velocity outside of the foam-like material along the corrugations than within the foam matrix itself. In a sheet metal packing, the liquid film flows as films on the surface of the sheets that are exposed to the gas flowing through the corrugations. In the case of foam, liquid, below some critical liquid flux, will flow as thin films on the struts that comprise the packing and those struts that are on the interior of the foam sheet are less effective in transferring mass with the gas flowing through the corrugation than it would be in the case of a sheet metal Therefore, in order to ensure that this wetted area is effectively used and to realize separation efficiency more consistent with the wetted surface area of the packing, the packing must be designed such that that at least some minimum portion of the vapor will flow through the foam-like material itself to contact liquid film along the struts and thereby realize the potential in such a packing.
As will be discussed, the present invention provides a method of distillation and a packing for use in such a method that will characteristically operate in a manner that will ensure vapor flow through the foam-like material itself that makes up a corrugated sheet.